Moving seismic source system for use in water-covered areas

ABSTRACT

A moving seismic source system for the use in water-covered areas comprising a conventional source and streamer cable means with multiple receiver, and additionally includes between the source and receivers a volume of the body of water containing a small percentage of free gas to form a gas-water mixture. The volume of gas-water mixture is of select size and shape to form an interface with the gas-free water to form, for example, a paraboloid which acts to reflect and attenuate acoustic waves that impinge on the interface and so minimizes extraneous seismic noise.

CROSS REFERENCE TO RELATED APPLICATIONS

This invention is related to my copending application Ser. No. 1,470,665filed on the same day as this application and entitled "Seismic SourceSystem for Use in Water-Covered Areas."

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention lies in the field of seismic sources for use inwater-covered areas. More particularly, it concerns a type of movingseismic source system which can be used advantageously in offshoreseismic prospecting, for recording seismic information with a minimum ofseismic noise.

2. Description of the Prior Art

A common type of acoustic wave detector system used in water-coveredareas comprises a single line of receptors or receivers (called astreamer cable) with the acoustic wave source positioned colinearly at aselected distance in front of the streamer. Acoustic wave energy movingupwardly and backwardly in the direction of the receivers causesformation of ghosts and horizontal waves which are recorded by thereceivers and which tend to mask the useful seismic signals detected bythe receivers.

SUMMARY OF THE INVENTION

It is a primary object of this invention to provide a moving seismicsurvey system for use in watercovered areas in which part of the waveenergy from the source is reflected and attenuated by a portion of thewater in which gas is injected to form a gas-water mixture of a selectedshape which is maintained in a geometrical relationship to the source.

It is a further object of this invention to utilize both the reflectionprocess and attenuation process to remove the seismic energy which wouldotherwise generate the noise.

It is a still further object of this invention to provide a means tomaintain a gas-water mixture within the body of water and partiallysurrounding a moving seismic source by the injection of gas into thewater at selected positions around the source and to move such gas-watermixture essentially in unison with the moving source system.

These and other objects are realized and the limitation of the prior artare largely overcome in this invention by means of a selected network oftubes or pipes which may be rigid or flexible; which have a plurality ofsmall perforations; and which are submerged in the water and suppliedwith air under pressure, so as to inject columns of bubbles of gas intothe water to provide the reflecting and absorbing volume of gas-watermixture.

While most of the gas-water mixture caused by the injection of the gasbubbles lies between the source and the streamer cable, some of thegas-water mixture must be positioned above the source to shield it fromthe surface. Thus, some of the gas is injected forward of the positionof the source so that with the rising position of the bubbles as afunction of time, there will be sufficient gas-water mixture by thesource so as to minimize energy reaching the surface which producesghost reflections.

An effective method of producing the gas-water mixture is by means ofair bubbles. Thus, a network of pipes with numerous perforations thatproduce air bubbles at selected positions is used. One embodimentinvolves a sector of a paraboloid with the seismic source at the focusof the paraboloid which trails the moving source. An extension on theupper and leading portion of the paraboloid sector provides bubbleemissions ahead of the axis of the paraboloid. In another embodiment, asingle planar arrangement of pipes, or multiple separate planararrangements can be used.

As the seismic source is shielded on the top and the back side by agas-water mixture of selected three-dimensional shape, the upwardly,outwardly, and backwardly going waves are reflected downwardly. Thus,only a minimum of acoustic wave energy travels to the surface of thebody of water to be reflected down as ghost reflections. Also a minimumof acoustic wave energy travels backwardly as a ghost wave within thewater layer.

BRIEF DESCRIPTION OF DRAWINGS

These and other objects and advantages of this invention and a betterunderstanding of the principles and details of the invention will beevident from the following description taken in conjunction with theappended drawings in which:

FIG. 1 illustrates schematically one embodiment of the inventionincluding the survey boat, the seismic source, the gas-water mixture,and the seismic streamer.

FIG. 2 illustrates in plan view, the structure which provides agas-water mixture of FIG. 1.

FIG. 3 illustrates a second embodiment in which the structure whichproduces streams of bubbles is one or more planar type systems ratherthan a partially circular system of pipes.

FIG. 4 illustrates a top view of one of the planar type systems of FIG.3. This may be made of rigid pipe or may be made of flexible plasticpipe so that it can be handled more readily in the narrow confines ofthe usual seismic ship.

FIG. 5 illustrates another embodiment in which the structure whichproduces the stream of bubbles is a shell of a shape different from thatof the other Figures.

FIG. 6 is a top view of the embodiment illustrated in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIGS. 1 and 2 there is shown schematically, inelevation and plan view, a moving seismic source and receiving systemwhich has been designed in accordance with this invention and is thepreferred embodiment. A body of water 17 is shown having surface 18 anda seismic survey ship 20 on the surface moving in the direction of thearrow 28 at a selected speed. Conventionally, such survey shipsgenerally travel at a speed of about 4 knots. At a selected distance 19,which may be in the order of 100 ft behind the ship 20, is positioned aseismic source 12 at a selected depth 15 which may be of the order of 40ft. The source may be suspended from floats (not shown but well-known inthe art) and towed by the ship 20 by connecting means which is shownschematically as the cable 24.

The oscillating bubble effect that accompanies the use of an explosivedevice or an air gun is an undesirable source of noise. This effect canbe eliminated by use of a water gun which ejects water rather thancompressed air, thereby eliminating the oscillating air bubble.

FIG. 2 shows a plan view of the structure 10 which generates thegas-water mixture that partially surrounds the source and occupies thespace above the source. This type of structure is further generallyillustrated in my copending application. Means for towing a structuresuch as structure 10 and source 12 at a selected depth in water behindthe towing vessel are well known.

In this particular structure there are a plurality of circular segments30 having perforations (shown for convenience as semicircles, althoughthey may be narrower or wider circular sectors). These circular pipesare attached to other pipes 36A, 36B, 36C, 36D, 36E, for example, tomaintain the proper three-dimensional structure and also to supply thecircular pipes with a compressed gas such as air which would bedelivered to the structure by means of an air hose associated with twocables 26 and air compressors on the ship 20. By making the pipesflexible, one can fold the structure when storing it on the ship.

In the particular embodiment illustrated, the top five semicirclesections of pipe are extended linearly as shown and indicated bynumerals 32N, 30O, 32P, etc. There are also shorter frontal sections34M, 34L, etc.

A vertical line 42 (FIG. 1) represents the axis of the paraboloid andthe position 13 indicates a focal point which is the same point as theposition of the seismic source 12.

The structure shown in FIG. 1 is moving in the direction of the arrows28 while the bubbles are rising vertically. Thus, from the point of viewof an observer on the structure 10, the gas bubbles formed by gasescaping through the perforations of pipes 30 would seem to be risingalong sloping lines such as 38 and 40. It will be clear, therefore, thatif there were no frontal extensions of pipe, there would be no bubblesdirectly above the source and therefore there would be no shieldingagainst the generation of ghost reflections immediately above thesource. Consequently, by using the frontal extensions 32 and 34, thereare sufficient bubbles injected into the water to provide a thick enoughlayer of gas-water mixture to provide the attenuation needed foracoustic wave energy moving upwardly from the source.

The paraboloid structure is very important in my copending application,where, because of the static situation, downward reflection of seismicenergy can be quite important to the recording of improved seismicrecords. In the structure 10 of FIG. 1 of this invention there is alsosome downward reflection of energy which firstly detracts from theamount that must be absorbed in the gas-water mixture above theparaboloid and secondly, provides additional downgoing energy whichwould be equivalent to a somewhat larger source.

The streamer cable 16 is shown schematically trailing from the ship 20,by support cables 22. The streamer cable is controlled by conventionalmeans to lie at a selected depth of water. The front end of such astreamer cable 16 is positioned behind the ship at a distance of some700-800 ft, and, therefore, for a given position all of the bubbleenergy from structure 10 would have arrived at the surface, and the gasand the gas-water mixture would be completely dispersed before thestreamer cable 16 would reach such given position. Thus, the streamer iscompletely receptive to reflection energy arising from the lowergeological interfaces, and is without ghost reflections or horizontallytraveling seismic waves in the water layer.

Referring now to FIG. 3 which is a schematic diagram similar to FIG. 1,but utilizing a different array of pipes and tubes having correspondingperforations which are supplied with pressurized gas through air hosesassociated with tow lines 26A and 26B. The structures 50A and 50B willbe described in terms of FIG. 4 but it can generally be said that theyrepresent a planar perforated pipe grill structure designed to produce aplurality of rising columns of small bubbles to provide a blanket orshield of gas-water mixture above and behind the source 13. The samewater gun source shown in FIG. 1 may be used in FIG. 3 and indicated bythe numeral 12. The source, the ship and the streamers are all moving inthe direction of the arrows 28. A set of short, dashed lines indicatedas 38B and 40B are shown and which correspond to the lines 38 and 40 ofFIG. 1. This would be the apparent angle of rise of the bubble for aselected speed of travel and size of bubble.

It is seen that the space above and behind the source 12 encompassedwithin the dashed lines provides a shield of gas-water mixture whichserves not only to reflect energy from the interfaces between thegas-water mixture and the body of gas-free water but also serves toattenuate acoustic waves generated by the source 12 which passes throughthese interfaces, so as to minimize those waves which travelhorizontally to the rear, outwardly and vertically to the surface, whichwould cause serious seismic noise.

Referring now to FIG. 4, there is shown one configuration of planarsystems 50A and 50B of FIG. 3. System 50A and system 50B would eachinclude a series of horizontal parallel pipes 52A, 52B, and 52C . . .52N. The ends of such parallel pipes are connected together by manifolds54A and 54B. The tensile members that carry the structure forward withthe ship are cables 56A and 56B which may be similar to cables 26A and26B. The compressed gas is supplied by corresponding compressors orother compressed gas source on the ship 20 which could be conventional.

Referring to FIGS. 5 and 6, there is shown another embodiment of themeans to produce the gas-water mixture. FIG. 5 represents a side view ofa curved rectangular pipe structure, and FIG. 6 represents the top viewof the embodiment of FIG. 5. There are shown curved end members, orpipes 60, which are connected by straight horizontal pipes 70A, 70B, . .. , 70N which have perforations 72 through which a gas such as air isinjected into the water in bubble column 74 in a manner similar to thatin the other embodiments. The shape of curved end members 60 is such asto get proper reflections of signals from source 12 so as to eliminateor greatly reduce ghost reflections from reaching the streamer cable.

The structures of FIGS. 1, 2, 3, and 4 which produce the gas-watermixture shield are shown as tubular members, rather than continuous,impervious sheets since the resistance to the movement of thesestructures through the water would be considerably less than for a largeimpervious sheet structure. In any event, the use of bubbles as done inthis invention provides configurations insuring that there will be asupply of bubbles above the source at the time the source is activated.

Other shapes of pipe structure can be devised which would produce avolume or shield of gas-water mixture of suitable size and shape thatcould be moved in synchronism with the ship, source, and streamer cable.

Small percentages by volume of air or gas in water can be utilized toform the gas-water mixture. For example, percentages by volume of gas aslow as 0.5% to 1% provides very large decreases in velocity and largeincreases in absorption of seismic energy and transmission through suchgas-water mixtures. Concentration of gas can increase to essentially100% if physically possible.

While this method can be used for any combination of liquid and gasmixture, it has been described in terms of air and water, solely forconvenience, and not in the way of limitation.

While the invention has been described with a certain degree ofparticularity, it is manifest that many changes may be made in thedetails of construction and the arrangement of components withoutdeparting from the spirit and scope of this disclosure. It is understoodthat the invention is not limited to the exemplified embodiments setforth herein but is to be limited only by the scope of the attachedclaim or claims including a full range of equivalency to which eachelement thereof is entitled.

What is claimed is:
 1. An apparatus which can easily be moved throughwater for both reflecting acoustic wave energy in a downward directionand attenuating upwardly and horizontally traveling acoustic wave energyproduced by a seismic source used in a body of water wherein the seismicsource and seismic sensor means are both moved substantiallyhorizontally through a body of water comprising:gas releasing meansinterposed in the water between the seismic source and the seismicsensor means and between the seismic source and the surface of the waterfor releasing gas bubbles into the water wherein gas releasing meanscomprises a plurality of coaxial concentric pipes arranged verticallywith spaces between adjacent pipes in the shape of at least a radialsector of a paraboloid, with a vertical axis of the paraboloid throughthe seismic source, each of said pipes having a plurality ofperforations; and means to supply said pipes with pressurized gas. 2.The apparatus as defined in claim 1 in which the radial sector comprises180° radial slice of the paraboloid.
 3. The apparatus as defined inclaim 1, including an extension means for extending the gas releasingmeans forward from the paraboloid sector.
 4. A method for bothreflecting acoustic wave energy in a downward direction and attenuatingupwardly and horizontally traveling acoustic wave energy produced by aseismic source used in a body of water wherein the seismic source andseismic sensor means are both moved substantially horizontally through abody of water comprising:interposing a gas releasing means in the waterbetween the seismic source an the seismic sensor means and between theseismic source and the surface of the water for releasing gas bubblesinto the water wherein the shielding gas releasing means comprises aplurality of coaxial concentric pipes arranged vertically with spacesbetween adjacent pipes in the shape of at least a radial sector of aparaboloid, with the vertical axis of the paraboloid through the seismicsource, each of said pipes having a plurality of perforations, and meansto supply such pipes with pressurized gas; and supplying gas to saidmeans to supply said pipes with pressurized gas.